1. Field of the Invention
The invention relates to a keyboard.
2. Brief Description of the Prior Art
As a result of the general trend from analog to digital systems and from hardware to software, key switches are used either individually or combined to keyboards in electro-technical equipment, communication equipment and last but not least in so-called terminals or data input systems. Numerous different designs and functional principles are available for such keyboards constituting the link between man and machine. A factor of great consequence to reliability and also to the contact between man and keyboard is the type of stroke. Apart from the so-called flat-type keyboards with a stroke up to about 1.5 mm, there are also keyboards with a stroke in the range between 1.5 and 6.35 mm, depending on the practical use. In addition to this type of keyboards, of course also keyboards with contact switches are in use. However, it is a characteristic common to all stroke-type keyboards that the operator by way of tactile sensation should be given a sort of feedback that the respective key has been actuated.
A keyboard is already known which comprises a plate of insulating material having a number of holes therein superposed on a printed circuit board provided with contact areas and conductor paths. Above the plate of insulating material there is a superposed metal plate comprising a large number of embossed dome-shaped protrusions, the overall arrangement being such that these protrusions are positioned above the holes. When any of the protrusions is pressed flat, an electrical connection between the contact points of the printed circuit board can be established for the time of pressing. As soon as the pressure has been released, the protrusion snaps back into its original position. In connection with this keyboard difficulties are likely to be encountered, especially in mass production, to always obtain a stop resistance being uniform at least within the keyboard and thus an equal tactile feedback to the user.
To eliminate this disadvantage, the hemisperical contact springs in another known keyboard were designed as loosely supported, separate plate springs. The plate springs are protected against lateral yield by a supporting plate of insulating material, in the holes of which they are positioned. Under the supporting plate a further plate of insulating material is provided, which likewise has holes exposing the contact points or areas. Under this latter plate the printed circuit board with the contact points and the conductor paths is positioned. Although the tactile feedback certainly is better in the case of this keyboard, just in mass production the insertion of separate springs is more difficult from the mounting aspect and more costly than the handling of a board provided with hemispherical domes.
Furthermore a keyboard is already known, wherein snap members are positioned above a printed circuit board provided with conductor paths, said snap members consisting of a rubber-elastic material and having the shape of truncated cones. The section above the contact points of the printed circuit board is connected to the remaining part by a folding wall. The construction of the wall is such that its thickness gradually decreases from an upper section to a central section and gradually increases from the central section to a lower section which is inwardly inclined into the interior of the snap member and extends from the central section to the upper section. By deformation of the wall thus an elastic force is obtained while the operator feels a sort of snap effect.
Furthermore so-called switch mats of non-conductive silicon rubber comprising a number of dome-shaped elevations are known for keyboards. In the hollow interior of such an elevation a contact element of conductive silicon rubber is arranged, which interacts with contact areas of a printed circuit board.
Because of the amount of shrinkage being up to 6%, depending on the processing parameters among other factors, and the resulting accuracy of manufacture, these switch mats can be manufactured with sufficient accuracy of positioning of the individual functional units with respect to one another or with the required reproducibility only in specific square dimensions or lengths, respectively.